The present invention relates to pointing devices for cursors on video display screens for personal computers and workstations, and more particularly relates to optomechanical sensors for translating rotation of a ball into digital signals representative of such movement.
Pointing devices, such as mice and trackballs, are well known peripherals for personal computers and workstations. Such pointing devices allow rapid relocation of the cursor on a display screen, and are useful in many test, database and graphical programs. Perhaps the most common form of pointing device is the electronic mouse.
With a mouse, the user controls the cursor by moving the mouse over a reference surface; the cursor moves a direction and distance proportional to the movement of the mouse. Although some electronic mice use reflectance of light over a reference pad, most mice use a ball which is on the underside of the mouse and rolls over the reference surface (such as a desktop) when the mouse is moved. In such a device, the ball contacts a pair of shaft encoders and the rotation of the ball rotates the shaft encoders, which includes a mask having a plurality of slits therein. A light source, often an LED, is positioned on one side of the mask, while a photosensor, such as a phototransistor, is positioned substantially opposite the light source. Rotation of the mask therebetween causes a series of light pulses to be received by the photosensor, by which the rotational movement of the ball can be converted to a digital representation useable to move the cursor.
In conventional electronic mice, a quadrature signal representative of the movement of the mouse is generated by the use of two pairs of LED""s and photodetectors. However, the quality of the quadrature signal has often varied with the matching of the sensitivity of the photosensor to the light output of the LED. In many instances, this has required the expensive process of matching LED""s and photodetectors prior to assembly. In addition, varying light outputs from the LED can create poor focus of light onto the sensor, and extreme sensitivity of photosensor output to the distance between the LED, the encoding wheel, and the photosensor.
There has therefore been a need for a photosensor which does not require matching to a particular LED or batch of LED""s, while at the same time providing good response over varying LED-to-sensor distances.
In addition, many prior art mice involve the use of a mask in combination with an encoder wheel to properly distinguish rotation of the encoder wheel. Because such masks and encoder wheels are typically constructed of injection molded plastic, tolerances cannot be controlled to the precision of most semiconductor devices. This has led, effectively, to a mechanical upper limit imposed on the accuracy of the conventional optomechanical mouse, despite the fact that the forward path of software using such mice calls for the availability of ever-increasing resolution. There has therefore been a need for a cursor control device for which accuracy is not limited by the historical tolerances of injection molding.
In addition, in some instances it is desirable to offer cursor control devices with different resolutions. Thus, for example, in some applications a cursor control device having a resolution of 200 dots per inch is appropriate, while in other applications a cursor control device having a resolution of 400 dots per inch is desired. In such circumstances, different mechanical components are needed to implement such different resolutions, leading to increased complexity and expense. This increased expense is necessarily passed on to the consumer, creating more expensive products. There has therefore been a need for an optomechanical implementation for a cursor control device which can operate at different resolutions, when combined with appropriate other components.
The present invention substantially overcomes the foregoing limitations of the prior art by providing an optical sensor employing a differential sensing arrangement. Such an approach, as described in greater detail hereinafter, substantially eliminates the need to match LEDs and the associated photosensors.
Further, by appropriately locating multiple sensors on a single substrate, and providing associated microprocessor control, it is possible to eliminate the need for a mechanical mask. Such elimination of the mechanical mask permits increased resolution by removing the constraints on accuracy associated with injection molding of plastics as compared to fabrication of semiconductors.
In addition, the sensor may comprise multiple sensors on a single wafer of silicon, permitting use at different resolutions simply by altering a single mechanical component and reselecting the sensors being monitored.
The present invention is also less sensitive to LED-to-sensor distances than the prior art.
The pointing device of the present invention, which is operable with electronic mice, trackballs, or other pointing devices which convert rotational movement to digital signals, includes at least one and typically two shaft encoders positioned to be rotated by movement of a rotational member, such as a ball. The shaft encoder includes a mask or encoding wheel having slits therethrough conforming to the resolution of the pointing device in dots per inch. Typical resolutions vary between two hundred and four hundred dots per inch, although substantially higher resolutions are not uncommon.
Positioned on either side of the encoding wheel for each shaft encoder are two pairs of LED""s and photosensors. The pairs are arranged not to be along a diameter of the wheel.
A differential sampling circuit detects motion of the wheel past the LED""s, which causes the generation of a quadrature signal. The quadrature signal is then provided to a microprocessor, where the signal is sampled and manipulated as described in U.S. patent application Ser. No. 07/717,187, now U.S. Pat. No. 5,256,913 , entitled Low Power Optoelectronic Device and Method and assigned to the assignee of the present invention. A conventional cursor control signal is then provided as the output of the microprocessor, although appropriate line drivers and related circuitry may be interposed. In particular, LED pulsing may be used to save power, among other techniques described in the aforementioned application.
In particular, the differential sensor of the present invention may be implemented as a single chip on which a plurality of photodetectors, such as photodiodes or phototransistors, may be disposed. In a typical arrangement, two pairs of photosensors spaced precise distances from one another are laid out on the semiconductor, although in a presently preferred embodiment, six sensors are fabricated into the semiconductor, with two of the sensors used only for low resolution, two used for both low and high resolution, and two used only for high resolution. In this manner the same sensor may be used for, for example, 200 dpi and 400 dpi resolution. In either event, two pairs of sensors are used at once.
The semiconductor bearing the photodetectors is positioned within the cursor control device so that the photodetectors are spaced apart from a pair of light source such as LEDs, with an encoding wheel placed therebetween. Depending on the desired resolution of the pointing device, the encoding wheel will have greater or fewer slots therethrough by which the light (usually but not necessarily infrared) is allowed to strike the photodetectors to indicate movement. Greater numbers of slots typically translates into increased resolution; the radial arrangement of slots about the center of the encoding wheel is precisely managed to ensure that light from the LEDs strikes the sensors only at the appropriate times; more particularly, a period of light striking one pair of detectors corresponds to a period of darkness at the adjacent photodetector. By this technique, the photodetectors permit current flow when the encoding wheel properly lines up, but are effectively open circuits when not struck by light. Accordingly, the output of the LEDs is a series of poorly shaped current pulses of different phase.
To provide improved detection, a comparator circuit comprising a plurality of current comparators receives on its A and B inputs the pulse train from a respective pair of photodetectors. In the presently preferred embodiment, the current comparators are fabricated on the same chip as the photodetectors, although such an arrangement is not in all cases required. In the presently preferred embodiment, four comparators are used, with the A and B inputs provided to each comparator being from selected ones of the six photodetectors. Each comparator then generates signal on a first output if A greater than B, and a signal on a second output if B greater than A. These outputs provide the inputs to an associated four RS latches, and in turn the output of the latches may be provided to a microprocessor for sampling and manipulation as required to provide an accurate representation of movement of the pointing device. In a presently preferred embodiment, the RS latches are also fabricated on the same substrate as the photodetectors and comparators.
It is therefore one object of the present invention to provide a cursor control device having optomechanical sensors which do not require matching of LEDs and photodetectors.
It is another object of the invention to provide a cursor control device using a ball having differential sensors for detecting rotational movement of the ball.
It is a further object of the present invention to provide a monolithic photosensor having a plurality of photodetectors disposed thereon for providing different resolutions of optical sensing.
It is yet another object of the present invention to provide a cursor control device which requires only an encoding wheel, a light source and a photodetector for detecting the rotation of a ball indicative of movement of the cursor control device.
These and other objects of the present invention will be better appreciated from the following Detailed Description of the Invention, taken in combination with the appended Figures.